A traveling wave tube and a klystron are electron tubes which rely on interaction of an electron beam emitted from an electron gun with a high frequency circuit for amplifying microwaves. As illustrated in FIG. 8, for example, these electron tubes are each composed of electron gun 131 for emitting an electron beam; high frequency circuit 15 for promoting the interaction of the electron beam emitted from electron gun 131 with a high frequency signal (microwave); a collector 16 for capturing the electron beam delivered from high frequency circuit 15; and anode electrode 136 for guiding the electron beam emitted from electron gun 131 into high frequency circuit 15.
An electron beam emitted from cathode surface of electron gun 131 is introduced into high frequency circuit 15 through anode electrode 136, and travels within high frequency circuit 15 while it interacts with a high frequency signal applied to high frequency circuit 15. The electron beam delivered from high frequency circuit 15 is applied to collector 16 and captured by a collector electrode included in collector 16. In this event, high frequency circuit 15 delivers a high frequency signal which is amplified through the interaction with the electron beam.
Electrons emitted from a side of cathode surface make defocus the electron beam of cathode surface. The defocused beam decrease the pass electrons to high frequency circuit, and decrease the amplification through the interaction with the pass electron beam.
For suppressing such influence of side emission, it is effective to dispose an electrical conductive “metal” in the vicinity of a side portion of the cathode.
In a case of an electron gun provided with no grid, the side emission can be suppressed by disposing a focus electrode, as the above-described metal, in the vicinity of a side portion of the cathode. However, in a case of a gridded electron gun, the grid needs to be disposed between the cathode and a focus electrode. Accordingly, in the case of a gridded electron gun, it is difficult to dispose the focus electrode in the vicinity of a side portion of the cathode.
As a method for resolving the above problem, a method of disposing a cathode shield, which is a metal, in contact with a side portion of the cathode is considered.
FIG. 9 is a cross-sectional schematic diagram showing a configuration in the vicinity of a cathode, of an electron gun 131y which is considered to be a general one of electron guns provided with a cathode shield.
The electron gun 131y includes a cathode 206, a metal 211, a cathode shield 221 and a heater 216.
The cathode 206 is heated by the heater 216 and thereby electrons from cathode 206 emit into a vacuum space.
Between the cathode 206 and the cylindrically shaped metal 211, a cathode shield 221, which is a cylindrically shaped metal is disposed. The cathode shield 221 is fixed to the cathode 206, by being inserted between the cathode 206 and the metal 211 or being welded to the cathode 206. Electrons emitted from a side surface of the cathode 206 are shielded by the cathode shield 221. Accordingly, the configuration shown in FIG. 9 can mitigate influence of the electrons emitted from the side surface of the cathode 206.
Japanese Utility Model Laid-Open No. H01-142148 discloses a power supply unit for grid-pulsed traveling-wave tube consisting of a heater power supply, a grid power supply, a collector power supply and a helix power supply.
Japanese Utility Model Laid-Open No. S59-146850 discloses an electron tube provided with an electron gun unit including a cylindrical ceramic outer shell component both of whose end surfaces each have a metal electrode attached to the surface, the metal electrodes being for applying a high DC voltage between them.
In the electron gun 131y shown in FIG. 9, the cathode 206 is in contact with the cathode shield 221, which is thermally conductive, as described above. Accordingly, thermal energy supplied from the heater 216 to the cathode 206 is transmitted to the cathode shield 221 and, thereby, thermal radiation of cathode shield 221 is increased. As a result, a problem that energy to be supplied to the heater 216, for heating the cathode 206, increases arises.